The present invention relates to a polymer-electrolyte membrane fuel cell comprising a laminate of such membrane, an electrode containing a catalyst, a porous, electrically conductive gas diffusion layer and a current collector plate having a gas distribution channel structure, the cell having a gradient of the gas permeability, which gradient is present at least in a partial area, in the laminate in the direction perpendicularly to the membrane, with a higher gas permeability closer to the membrane an a lower gas permeability closer to the current collector plate, wherein in operation at the membrane by a hydrogen-oxygen reaction water and heat are produced. The invention further relates to a method for manufacturing such fuel cell.
The polymer-electrolyte membrane fuel cell or PEM fuel cell is operated with the reaction gases hydrogen and oxygen or air. According to a known structure (EP 0560295A), it consists of, arranged from the outside to the inside, two current collector plates with gas diffusion layers, two electrodes containing the catalyst and an ion exchange membrane located between the electrodes, with the ion exchange membrane and the electrodes forming the membrane-electrode-unit (hereinafter referred to as MEAxe2x80x94membrane electrode assembly). Typically the current collector plates contain structures than extend parallel to the MEA for supplying and distributing the reactants across the entire surface of the cell. Since the voltage of each individual cell is much too low for any practical applications, a multitude of these cells must be serially arranged. In the resulting fuel cell pile or fuel cell stack, the coinciding current collector plates are replaced with so-called bipolar plates whose function consists of directing the current though the stack and of isolating the reaction gases.
By supplying hydrogen which is a typical reaction gas to the fuel cell anode side which is positioned in a gas compartment sealed off to the outside, cations are generated on the anode side catalyst layer which cations diffuse though the ion exchange membrane. Simultaneously, the electrons that were also produced are directed via an outer electric circuit with a load resistor from the anode to the cathode. The supplied oxidation agent is oxygen, and preferably the oxygen concentration in ambient air is sufficient; this oxygen is now reduced in the cathode by way of reception of the hydrogen ions and electrons. Water results as a reaction product. The reaction enthalpy is released in the form of electric energy and dissipated heat.
An essential problem in this process is the water economy of the fuel cell. In the initially mentioned known fuel cell (EP 0560295A) the water economy particularly in the electrode layer which contains the catalyst is influenced by means of hydrophobic coatings covering the catalyst carrier which itself is covered by the catalyst. In order that the catalyst still comes into contact with the reactants, in this layer the porosity increases towards the membrane. In the area of low porosity a higher percentage, in the area of the high porosity a lower percentage of the catalyst is inactive. The water generation takes place over all of the thickness of the layer and can also deactivate parts of the catalyst by flooding. On the other hand, the membrane is able to perform under optimal conditions, i.e. it conducts the hydrogen ions optionally, only if it contains a sufficient amount of moisture. If the moisture content drops too low, the internal resistance of the cell increases considerably due to the increased membrane resistance, thereby reducing performance. Thus, for an optimal cell operation at a given temperature it is necessary that the air""s humidity is at almost 100 percent at each and every place of the membrane. If the cathode gas air flows through the distribution channels of the current collector plates and diffuses through the gas diffusion layer, it has a low partial pressure with regard to water vapor upon entering the gas compartment, and a high one upon exiting because oxygen reacts to form water at the cathode. The diffusion flow between the membrane""s surface and the distribution channel, which is caused by the partial pressure differences of hydrogen, dries the membrane at the entry point of the cathode gas, and at the exit point, on the other hand, water deposits may occur in the diffusion layer. At a given operating temperature and in order to compensate for differences in the water balance of the membrane it is therefore necessary to achieve a composition of the cathode gas that is as constant as possible across the membrane""s surface; the same applies for the anode gas.
With conventional methods this problem is only partially solved using external humidification systems, at times in combination with cooling systems, which systems, by way of measuring the membrane moisture at least in intervals, provide for water balance adjustments of the cell. It is a disadvantageous aspect of these humidification systems that they place an additional burden on the fuel cell system in terms of internal energy consumption and also weight, which is particularly undesirable for their application in small, portable systems, as well as in terms of additional cost thereby reducing the competitiveness of the fuel cell in comparison with conventional energy supply systems. Furthermore, the above solution does not address the problem of how to achieve an even gas distribution on the surface of the catalyst and of the cell membrane. The objective is to operate a fuel cell without humidifying the reaction gases.
Electrode-catalyst layers which are porous and such allow some gas diffusion are also known from WO 97/20359. These layers may also consist of a laminate of several films and can be reinforced by a conductive grid. However, they do not have a gradient of the gas permeability.
U.S. Pat. No. 5,641,586 shows one solution as to how to achieve a uniform distribution of the reaction gases. It provides that two layers are arranged between the electrode catalyst layer and the current collector plate; adjacent to the MEA is a macro-porous, hydrophile gas diffusion layer, and adjacent to the current collector plate is a macro-porous, hydrophilic flow field. The flow field has two intermeshing channel structures, on the one hand, for distributing the reaction gases at the gas diffusion layers and, on the other hand, for removing the reaction products. Although this apparatus allows the even distribution of the reactants across the surface of the membrane and water deposits are also avoided by way of the application of a hydrophobic layer, the danger of drying continues to remain a threat because the gas diffusion layer does not prevent the reaction product from exiting even if the membrane has an insufficient moisture supply.
One way for maintaining the water economy of a fuel cell on a constant and optimal level is shown in DE-OS 14 96 172, according to which a water diffusion electrode made of a palladium/gold alloy is used which is permeable for hydrogen but prevents the penetration of fluids. However, this design is not suitable for use at the cathode, where the problem of having to compensate for water imbalances primarily is since this electrode is not permeable for oxygen.
Thus, the object of the present invention is to replace the conventional gas diffusion layer of the PEM fuel cells with a gas diffusion structure whose diffusion properties are dimensioned in such a way that the water balance at the MEA is always uniform.
The fuel cell of the present invention is to be manufactured by a simple, inexpensive method for producing such gas diffusion structure.
According to the invention these objects are accomplished by a gas diffusion structure which is characterized in that the gas permeability gradient exists in the gas diffusion layer which is adjacent to the electrode containing the catalyst, and that at least in said partial area of the gas diffusion layer the gas permeability closer to the current collector plate is lower to such a degree than in the vicinity of the membrane that a gas composition occurs which over the surface of the membrane is approximately constant, and the water which at the operating temperature is generated in vaporform creates such a water vapor diffusion stream through the gas diffusion layer that such a humidity content of the membrane which is optimal for the conductivity is materially being maintained, and by a method for producing the gas diffusion structure, which method is characterized in that the gas diffusion layer is produced with a gas permeability increasing in the direction toward the membrane. The gradient is thus provided for in the gas diffusion layer which for the purpose of gas diffusion must have a certain thickness and wherein the generation of the water takes place beyond the delimiting face oriented toward the catalyst. The water vapor defuses in an administered stream of vapor out of the inner zone of the cell in such a manner that along all of the membrane approximately equal gas access conditions and humidity conditions exist which conditions can be adjusted to be optimum.